(ε) is the
fractional deformation resulting from stress. It is measured as the
ratio of change in some
dimension of a body to the original dimension in which the change occured.
Strain = change in dimension / original dimension
Thus, the normal strain under an axial load is the change in length ΔL over the original length L0:
ε = ΔL / L0
Strain has no units because it is a ratio of like quantites. There are, however, exact definitions for various situations which will not be discussed in this writeup.